Modification of the lunar surface by the solar-wind bombardment

Planet. E&ace Sd. 1963. Vol. 11. pi. 1257 to 1261. Pqamon

Pra

Ltd. Printed in Northern Irehad

MODIFICATION OF THE LUNAR SURFACE BY THE SOLAR-WIND BOMBARDMENT* G. K. WEHNER, C. E. KRNKNIGHT and D. R-G General MI&, Inc., Electronics Division, Minneqolii, Minnesota (Received 21 June 1963)

r--tigduration,

solar-wind sputtering conditions were simulated at a much acceler-ted hydrogen ion beams or in low-presnue, noble gas or hydrogen plasmas. Experiments with metal targets and metal-, oxide-, and rock-powder samples dunonstmte the leveling and smoothing of macros4xpic surface features, and cementing togetlm of loose par&S into a porous, brittle, fibrous crust. Certain oxide surfaces become emiched with metal atoms under the bombardment and sputtering action. It is concluded that many of the umuual proof the lunar surface can be explained by the action of solar-wind bombardment. 1.INTRODUCTION

By simulating solar-wind conditions in the laboratory, we were able to give fairly reliable estimates of the sputtering rates for an unprotected body like the Moon(l) in orbit around the Sun. These rates are small, but not negligible for long bombardment times. We obtained, for a proton flux of 2 x loS/cms set and a superimposed a-particle flux of 0.3 x 10s/cms set at 600 kmlsec particle velocity, rates of 4.3 A/yr for Fe, and O-4 A& for stony materials. In agreement with previous estimates by ReXeP it appears that the He component in the solar wind is more important to sputtering than the proton component. If the solar wind blew with the same intensity over the period of the Moon’s existence, one fmds that the Moon must have lost a layer of roughly 20 cm thickness in 4-5 x 109yr. This is of the same order as estimated for the accretion of micrometeoric materiaLa) From measurements of the velocity distribution of sputtered atoms, we conclude that many of the sputtered atoms have velocities which are higher than the escape velocity of the Moon(l). In the course of those sputtering measurements in which we have simulated solar-wind conditions with mass-separated, hydrogen-ion beams or in low-pressure, noble-gas or hydrogen plasmas, a variety of observations with regard to modifications of the bombarded target surfaces were made. From these, one can draw some interesting conchtsions as to structural and compositional changes which must have taken place over the millenia on the hmar surface. 2. EKP-

When material is sputtered from a polished metal surface, the target becomes rough on a microscopic scale because differently orientated crystallites are sputtered at different rates and grain boundaries as disturbed regions usually appear as furrows. Further, the surface becomes covered with etch pits, but these cannot be correlated with the original dislocations in the lattice(d). Obviously the bombardment causes considerable rearranging of single dislocations in the region close to the surface. Imbedded insulating or low sputtering rate particles can shadow the material underneath and leave hillocks or spires which extend up to the height of the original surface. Occasionally protrusions or spires are found which extend above the original surface. These probably grew from surface-migrated or sputteredand-reattached atoms. A m8croscopic8uy rough surface becomes smoother under sputtering action because * This work was supported under Contract NASw-424 from the National Aeronautics and Space Administratl~. 1257

1258

G. K, WE!?INE&C. E. EGG

and D. R~~ERG

many atoms are dislodged and transported to the deeper lying areas of the target surface. This happens because atoms sputtered fern a flat surface by obliquely incident ions are preferentially ejected iu a forward direction. Material sputtered from the &U&Sand sides of protrusions on a rough surface is therefore pushed into holes and crevices. This phenomenon was demonstrated by sputtering a metal screw in a low-pressure plasma. The threads are seen to become more and more shallow as sputtering proceeds. (a) Experimentswithan assemblyofmull steel balls A number of interesting phenomena can be demonstrated when one bombards and sputters a dish filled with small, closely-packed metal spheres. We performed such experiments with 0975mm dia., steel balls in a low-pressure, Hg plasma. The Hg plasma was created in a grid-stabilixed, d.c., vacuum arc between a Hg-pool cathode and an anode, in the geometry of much of our previous sputtering work? The discharge data were: arc current, 4 A; arc voltage, -25 V; Hg gas pressure, 10-8 torr; target at minus 500 V with respect to plasma; ion current density at target, -10 mA/cma; thickness of ion sheath covering the target, several mm; under the bombardment the target assumes a temperature of -300°C. Compared to the solar wind, one has here a much higher flux density and a much higher sputtering rate. In fact, an estimate shows that one hour of laboratory sputtering under these conditions should be equivalent to several million years of solarwind sputtering at the lunar surface. Figures l-4 show various stages of progress in sputte~ng this sphere assembly. The observations can be sod as follows: 1. With the ion-accelerating sheath larger than the sphere diameters, the surface plane is essentially under normal incident bombardment. Individual spheres experience normal ion incidence at the leading top area, but oblique incidence at the sides. Sputtering rates under oblique incidence are higher than under normal incidence and for this reason the surface spheres assume a conical shape (Fig. 2), as previously observed with single Fe or MO spheres sputtered in a uniform ion beamf6). 2. The ions are able to enter into the assembly more deeply at places where the top layer leaves holes between touching spheres. The lower spheres are then sputtered in areas which are sharply defined by the “masking” of the upper spheres. Such spheres are photographed in Fig. 5. 3. Many sputtered atoms are ejected in a forward direction and pushed deeper into the assembly of spheres. These atoms build up deposits which ~n~bute not only to the leveling of the surface, but which at the same time provide the “cement” to seal touchmg spheres together. A layer of spheres containing spheres from the second and even the third layer can then be lifted off as a crust after the target has been sputtered for a sufhciently long time (Fig. 6). 4. After the top layer of spheres has been sputtered off completely, which in our experiments took about 100hr, flaky, porous deposits appear at places between the spheres. They look like a dark mold (Fig. 3) but when ilhuninated and viewed from the side they turn out to be shiny, closely spaced spires and flakes with a metallic appearance. They probably originate from flakes and remnants sputtered free and aligned, normal to the target surface, by the electric field and the ~rn~~~t. The fact that this “mold” stands out from the su.&aceindicates that this caption must have a low sputtering yield. This is not surprising because high surface roughness favors the recapture of sputtered atoms.

Frc. 1. ASSEMBLY OF STEEL BALLS BEFORE SPUTTERING.

Ffci.2. SAMETARGETAFTER

16 hr OFSPUTTERING BY 500eV CURRENT DENSITY.

Hg

IONSAT -IO

mA/cm~ 125s

FIG, 3.

SAME TARGET

2.5

X

AFTER

lOa yr

120 hr

OF SPiJTTERfNG

OF SOLAR-WIND

FIG. 4.

SIDE VIEW

UNDER

SPUTTERING

SAME CONDITIONS:

AT THE

OF SAME TARGET

LUNAR

AS IN FIG.

SURFACE.

3.

EQUIVALENT

TO

FIG. 5. SECOND

FIG.&

ASSEMBLY

LAYER SPHERES SPUTTERED ONLY IN PLACES WHERE LEAVES OPEN SPACES.

OFCEMENTED

TOP LAYERSOFSPHERES

TOP LAYER

OF SPHERES

LIFTED FROMTARGETSHOWNINFIG.

3.

FIG.~. SELF-SUPPORTINGSURFACECRUSTL~FTED FROMTARGETOFPOWDERED IONBOMBARDMENTEQUIVALENTTO 1.5 X 10*yrOFSOLAR-WIND

BASALTAFTERH~' BOMBARDMENT.

FIG,~. CROSS-SECTION VIEW OF PIECESOF CRUST FORMED ON CU~OAFTER BOMBARDMENT HYDROGENIONSEQUIVALENT TO ROUGHLY l@yrOF SOLAR-WIND ACTION.

WITH

hC;. 9.

~RLIQUE

VIEW

OF SAME

cll,o

SURFACF

CRUST

AS IN

FIG.

8.

MO~~ICA~ON

(b) ~~eri~ts

OF THE LUNAR SURFACJZBY SOLAR-WIND ~~~~~

1259

with metal powders

In sputtering of metal dust under similar discharge conditions, the following observations were made: 1. The metal dust (Fe or MO) has originally a dull greyish appearance. The surface assumes a shiny metallic appearance after sputtering. Oxide films are obviously removed and the exposed surfaces become atomically clean. 2. A brittle crust is formed after more than 40 C/cm%of sputtering. This crust can be lifted off in pieces or as a whole and the dust particles hold together well. The crust is thicker at places where the dust was more loosely packed, because the sputteriug action goes deeper there. 3. The crust, when sectioned, has a fibrous structure, covered with needles and spires which are aligned in the direction of the ion born~~~t. No difference exists between the behavior of Fe and MOpowder. This excludes the possibility that any melting effects are involved in the crust formation. In fact, the smface barely reaches the temperature of melting lead, as an experiment with lead dust showed. (c) Experiments with compound and tnsulating powders With insulators, a di@ulty arises in maintaining an electric field to accelerate the ions towards the surface because of the charge which accumulates at the surface. Recently a method employing radio-frequency fields was developed”) to solve this problem. Bombardment and sputtering are accomplished iu a low-pressure, d.c. plasma by applying a ~~-~uen~ voltage to a metal electrode which is covered with the insulator material to be sputtered. A convenient frequency for this operation lies in the 1 to 10 MCrange. The target in our experiments consisted of a quartx dish (with a metal bottom as the electrode) filled with a 2-mm thick layer of the powder to be sputtered. From the thickness of the dark ion sheath which covers the target. one can estimate that the maximum bombarding ion energy (the energy, of course, ranges between zero and the maximal value) was of the order of SO0eV at the observed 5 mA/cmBcurrent density. Materials studied thus far include Also,, CuO, C&O, FeO, Fe*O,, and various rock powders. The results can be summarixed as follows : 1. The surface of many materials, after sufllcient bombardment (> 100 C/curs), becomes covered with a brittle crust in which the individual dust particles become cemeuted together by sputtered atoms. Figure 7, for instance, shows that in the case of powdered basalt, a whole cemented surface crust can be.lifted from the dish. In A&O, only very little or very fragile crusting is observed. The crust is thicker in more loosely packed places or materials. 2. The surface layer of many compounds becomes enriched with metal atoms. Black CuO is first converted into red Cu,O before becoming covered with a very porous Cu layer. The red FesOs converts into FesO,, then to ferromagnetic FeO, and finally Fe metal traces appear on the surface. The reduction to a composition richer in metal and the formation of a “metal black” causes a darkening of the surface in most cases. Obviously, in the process of breaking up molecules under the bombardment and of sputtering atoms back and forth, oxygen atoms are more likely to escape from the surface than metal atoms. 3. As previously noted for metal powders, whenever a crust is formed it has a fibrous structure with closely spaced needles and spires and deep smah holes all aligned in the direction of the ion ~rn~ent. The pred ominance of ~c~~pi~ly very steep walls is evidenced by the fact that the surface looks rather dark when viewed in isotropic light, but becomes shiny when illuminated and viewed from the same oblique direction.

1260

0. K. WJNNER, C. E. KENKNIOHT and D. ROSENBERO

(d) Experiments

in hydrogen

plasmas

The Hg-bombardment experiments described so far give an enhanced picture of the physical sputtering effects such as arise in the solar wind from He ions or c+particles. In the case of hydrogen ions, chemical effects may become superimposed and, therefore, as a next step to closer solar wind simulation, similar powder-bombarding experiments were performed in a hydrogen discharge. In this case the plasma was excited with high frequency fields. The operating data were as follows. Hydrogen gas pressure controlled with Pd leak and pump throttle: lO-* torr. Background impurity pressure in bakeable tube: lOA torr. Excitation of hydrogen plasma: 50 MC 1 kW transmitter, inductively coupled by a tuned external ribbon wrapped around the tube. Sputtering of target : 2-5 MCas in d.c. Hg plasma, ~rn~g ion current density 2 ~~~=, maximum ~rn~ng ion energy 800 V. In this discharge, one has an undetermined ratio of H+, Ha+and Hs+ ions. With the sputtering yields about 2 orders of magnitude lower than in Hg, one needs correspondingly longer sputtering times for obtaining visible effects. In most experiments the bombardment amounted to > 100 Clcma. Although chemical reactions between oxygen and hydrogen (with water molecular formation) may now become superimposed on physical sputtering, it turns out that the visible effects, with respect to crusting, reduction of oxides, darkening etc., are very similar to those found for Hg-ion bombardment. Figure 8, for instance, shows pieces of the crust in cross section formed on CusO powder. The thickness of this crust was here about O-2mm. Figure 9 shows an oblique view of the same surface. 3. CONCLUSIONS

With respect to the lunar surface the following conclusions can be drawn: 1. Sputtering by the sohu wind must have caused some leveling and smoothing of surface features. The rate of this leveling is probably somewhat larger than the sputtering rate. We estimate that a narrow surface protrusion, !I0cm in height, could be completely leveled within the lifetime of the Moon. 2. Loose dust particles must have become somewhat crusted together under solar-wind conditions. With micrometeorites stirring up and turning the lunar surface material over continuously, one can assume that the crusting is not confined to the outermost surface layer but that over the millenia a very porous layer of brittle material with very low density has been built up. In this connection 6pik ’ s m) estimate that a complete turnover of the upper l-cm layer by the action of ~~orne~~~s would take place in about 10’ yr is of interest. We estimate that the layer in which solar-wind sputtering played an important part is probably several decimeters thick. 3. Oxides and other compounds became enriched with metal atoms. Such a possibility was recently discussed by Buettner@~in connection with certain discrepancies in the electrical conductivity of the lunar surface. The metal atom enrichment is partly responsible for the observed darkening of our bombarded samples. In the process of the reduction of oxides with hydrogen ions, water molecules may have been formed; some of these may have been trapped below the surface. 4. In our experiments, the surface crust assumes a fibrous structure with closely spaced needles and spires and deep small holes all aligned in the direction of bombardment. At the Moon, the conditions are different because the ~rnb~~t sweeps the surface over a range of angles. This should cause the surface to become more complex and irregular, but it probably would not change its basic character. This layer is probably not contradictory

MODIFICATION

OF THE LUNAR SURFACE BY SOLAR-WIND BOMBARDMENT

1261

to the “fairy castle” structure proposed by Hapke’lO) or the “skeletal fuzz” proposed by Warren’ll) for explaining the photometric properties of the lunar surf&e, 5. Heavy metal atoms are favored for being retained and enriched for two reasons: The ejection velocities of heavy atoms are lower than those of light atoms and a higher proportion of these will fall back to the Moon. Heavier atoms furthermore have lower sputtering yields than light ones under light-ion bombardment on account of the poorer energy transfer. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8.

G. K. Wmmmt, C. E. KENKNI~~~ and D. L. ROBENBBRO, Planet. S’ce Sci. 11, 885 (1963). L. REIFFEL,Nature, Lord. 185,821 (1960). E. J. &UC, Planet. Space Sci. 9,211 (1962). G. K. WHHNER, J. Appf. Phys. 29,217 (1958). G. K. WEHNBR,Admttxs in Electronics and hktron Physics, 7,239 (1955). G. K. Wmmm, J. Appl. Phys. 30,1762 (1959). G. S. hDBBSON, W. N. MAYER and G. K. WEHNER,J. Appl. Phys. 33,299l (1962). E. J. C)PIK,Progressin the Astromuticul Sciences. Vol. 1, p. 219. North Holland Publ. Co., Amsterdam (1962). 9. K. J. K. BUBTTNBR, P&met. Space Sci. 11, 135 (1963). 10. B. W. HAP~E. Proceediqqs of the Lmar Surface Materials Conference (1963). To be published. 11. C. R. WARREN,Science 140,188 (1963). hWIoaat%-hfOJ&eJIKpOBaJIJIHCb ~OJIl'OBpe?deHHbIe yCJIOBHK paCIIHJIeIiEfI COJIKpKHM BeTpOM Ka BblCOKO ycKopeKKoaa racma6e aly¶aMH BOAOp07gib1X HOHOB, CeIIapEpOBaKKbIX no Macce, HJIA B mae~a~ 6~1aropomzmx ra6oB KJIH BoAopona non KKBHHM ~aB.meKtieM. OIIHTH c ruemmmecmm mmemm si o6pasQam mem.m.m~ecmx H o~c~~lrmrx IIOpOIIIKOBE lIOpOIJ.lKOB l'OpKHX IIOpOn lIOKaWIBaIOT BhIpaBHKB;LHEe E CIVWKHBaKEe MaKpOCKO~E~eCKEXKOBepluIO~~X¶epTH~eMeET~~~~~EX~aCTH~BIIOpHCTyIO, XpyIIKyiO, BOJIOKHHCTyio KOpy. HeKOTOpHe OKCEzpIhIe IIOBepXKOCTE 060~~a10TCK HeTaJIJIH~eCKWMH aTOHaME BOA ~eih2TBEeP.f6oaa6aplUrpomf H pilC~eHEri. C~eJi~ BHBOA, ¶TO MHOI’Oe E0 HeO6blKEOBeHHhnc CBOikCTB JIyHHOti IIOBepXliOCTK MOKSHO 06'bJiCHKTbJJetiCTBKeM6oM6apwposm COJIK~HIU BeTpOM.